A chemical absorption method is used in a decarbonation process performed in existing large-scale plants for hydrogen production, urea production, or the like. To separate CO2, such a chemical absorption method requires a huge CO2 absorption tower and a huge tower for regenerating a CO2 absorbing liquid. The step of regenerating the CO2 absorbing liquid also wastefully consumes energy because it requires a large amount of steam to remove CO2 from the CO2 absorbing liquid by heating for the reuse of the liquid.
In recent years, as a countermeasure for global warming, natural energy that does not emit CO2 has been expected to come into wide use, but natural energy has a significant problem in terms of cost. Thus, attention has been paid to a method called CCS (Carbon dioxide Capture and Storage) in which CO2 is separated and collected from waste gases from thermal power plants, ironworks and the like, and buried in the ground or sea. Currently, even CCS is based on application of the chemical absorption method. In this case, for separating and collecting CO2 from thermal power plants, not only large-scale CO2 separation equipment is required, but also a large amount of steam should be fed.
Besides separation and collection of CO2 from waste gases from thermal power plants and the like, for example, purification of natural gas as a major energy source requires separation of CO2 because it contains several % of CO2 in addition to methane as a main component. A chemical absorption method is used in existing processes for such purification.
On the other hand, a CO2 separation and collection process using a membrane separation method is intended to separate a gas by means of a difference in velocity of gases passing through a membrane using a partial pressure difference as driving energy, and is expected as an energy-saving process because the pressure of a gas to be separated can be utilized as energy and no phase change is involved.
Gas separation membranes are broadly classified into organic membranes and inorganic membranes in terms of a difference in membrane material. The organic membrane has the advantage of being inexpensive and excellent in moldability as compared to the inorganic membrane. The organic membrane that is used for gas separation is generally a polymer membrane prepared by a phase inversion method, and the mechanism of separation is based on a solution-diffusion mechanism in which a gas is separated by means of a difference in solubility of the gas in the membrane material and diffusion rate of the gas in the membrane.
The solution-diffusion mechanism is based on the concept that a gas is first dissolved in the membrane surface of a polymer membrane, and the dissolved molecules diffuse between polymer chains in the polymer membrane. Where for a gas component A, the permeability coefficient is PA, the solubility coefficient is SA, and the diffusion coefficient is DA, the relational expression: PA=SA×DA holds. The ideal separation factor αA/B is expressed as αA/B=PA/PB by taking the ratio of permeability coefficients between components A and B, and therefore αA/B=(SA/SB)×(DA/DB) holds. Here, SA/SB is referred to as solubility selectivity, and DA/DB is referred to as diffusivity selectivity.
Since the diffusion coefficient increases as the molecular diameter decreases, and the contribution of diffusivity selectivity is generally greater than that of solubility selectivity in gas separation, it is difficult to allow selective passage of gases having a larger molecular diameter by suppressing passage of gases having a smaller molecular diameter among multi-component gases having different molecular diameters.
Thus, studies are conducted on a permeable membrane called a facilitated transport membrane that allows selective permeation of a gas by a facilitated transport mechanism, in addition to a solution-diffusion mechanism, using a substance called a “carrier” which selectively and reversibly reacts with CO2 (see, for example, Patent Document 1 below). The facilitated transport mechanism has a structure in which a membrane contains a carrier which selectively reacts with CO2. In the facilitated transport membrane, CO2 passes not only physically by the solution-diffusion mechanism but also as a reaction product with the carrier, so that the permeation rate is accelerated. On the other hand, gases such as N2, CH4 and H2, which do not react with the carrier, pass only by the solution-diffusion mechanism, and therefore the separation factor of CO2 with respect to these gases is extremely high. Energy generated during the reaction of CO2 with the carrier is utilized as energy for releasing CO2 by the carrier, and therefore there is no need to supply energy from outside, so that an essentially energy-saving process is provided.